1. Field of the Invention
The present invention relates to a method and a device for judging a characteristic of a tire. In particular, the present invention relates to a method and a device for judging the degree of running stability performance of a tire, which represents the degree of handling stability and controllability of a vehicle when the vehicle, on which the tire to be judged is mounted, is driven.
2. Description of the Related Art
Today, in response to enhanced motion performance of automobiles, tires to be mounted on the automobiles are requested to have higher running stability performance of the tire. The running stability performance of the tire corresponds to the degree of running stability obtained when a vehicle, on which the tire is mounted, is driven. The degree of running stability obtained when the vehicle is driven is represented by the degree of handling stability and controllability felt by a driver who drives and handles the vehicle. Specifically, the running stability of the tire is an index for the feeling of a driver of a vehicle, and is basically evaluated by a subjective evaluation carried out by the actual steering of a vehicle (on which tires are mounted) which is driven and steered by a test subject (actual vehicle steering feeling test). For the actual vehicle steering feeling test, however, time and equipment (a vehicle, a driving course and the like) required for the evaluation are enormous, resulting in need of high cost. For this reason, a method of evaluating the running stability performance of the tire without carrying out the costly actual vehicle steering feeling test has been conventionally demanded.
Conventionally, the running stability performance of the tire has been obtained by, for example, using known drum type or flat belt type indoor cornering test devices. To carry out the evaluation of the running stability performance of the tire by using the cornering test device, for example, a slip angle is applied to a tire while the tire is rolling on a road surface of the cornering test device at a constant rolling speed. In this manner, a frequency response characteristic of a cornering force at the rolling speed is obtained. The known indoor cornering test device is used to obtain an absolute value of the cornering force generated in the tire, which serves as a measure for determining the dynamic performance of the tire. The frequency response characteristic of the tire is also used for evaluating a transient response characteristic of the cornering force in response to a variation of slip angle. In general, the controllability of the vehicle is improved and the degree of running stability felt by the driver of the vehicle becomes higher as the cornering force generated in the tire becomes greater. Conventionally, the degree of the cornering force is obtained mainly from the test result obtained by the indoor cornering test device. Then, the running stability performance of the tire is evaluated according to the degree of the cornering force.
In order to ensure the running stability of the vehicle, Japanese Patent Application Laid-Open No. 2003-226259 (hereinafter, referred to as Patent Document 1) describes a method and a device for adjusting an alignment of a vehicle, for adjusting the alignment such as a toe angle or a camber angle of each wheel mounted on a vehicle to allow a stability factor to be equal to or higher than a reference value. The stability factor represents the running stability of the vehicle mainly when the vehicle runs straight. Patent Document 1 proposes the alignment adjustment method in consideration even of the effects of a variation of lateral force generated by the application of a disturbance such as the irregularity of a road surface (paragraph [0005] in Patent Document 1 and the like).
Specifically, when the tire rolls on a road surface having irregularity in contact therewith, the tire is deformed by a variation of load generated by the vertical movement of the road surface relative to the tire. This deformation of the tire varies a lateral force having load dependency, called plysteer, which is caused by a structural factor of the tire, a lateral force having load dependency, called conicity, which is caused for manufactural reasons, and a lateral force generated by applying the slip angle (toe angle) to the wheel. Since the variations in lateral force greatly affect the running stability, the alignment adjustment method in consideration of the effects of the variations in lateral force has been proposed.
More specifically, the tires, which are mounted on the vehicle, are placed on a tire-driving surface having a plurality of protrusions. Then, the tire-driving surface is driven. Each of the tires rolls on the tire-driving surface while being caused to pass over each of the plurality of protrusions. During the rolling, a variation of tire lateral force generated by the deformation of the tire, which is caused by running over the protrusion and running down from the protrusion, is detected by a sensor. In this manner, the variation of tire lateral force generated by the deformation of the tire is detected for each of the tires to obtain the maximum value of the detected amount of variation of tire lateral force (lateral force variation amount). Based on the lateral force variation amount of each of the tires, the alignment such as the toe angle or the camber angle of each of the wheels mounted on the vehicle is adjusted to set the stability factor, which represents the running stability of the vehicle mainly when the vehicle is running straight, to the reference value or higher.
According to the above-mentioned conventional method for evaluating the running stability performance of the tire by using the indoor cornering test device, however, the result of evaluation of the running stability performance of the tire, which is obtained by using the indoor cornering test device, is not identical with that obtained by the actual vehicle steering feeling test at good accuracy. The result of evaluation of the running stability performance of the tire, which is obtained by the actual vehicle steering feeling test, corresponds to numeric information representing the degree of running stability actually felt by a driver who drives a vehicle when the vehicle, on which a tire to be evaluated is mounted, is actually driven.
The method described in Patent Document 1 has an object of adjusting the alignment such as the toe angle or the camber angle of each wheel mounted on the vehicle to allow the stability factor representing the running stability of the vehicle mainly when the vehicle runs straight to be equal to or higher than the reference value. Although the method described in Patent Document 1 can adjust the alignment of each of the wheels mounted on the vehicle, the running stability performance of each tire cannot be judged. If the running stability performance of the tire is varied for each tire mounted on the wheel, there is a possibility that the alignment such as the toe angle or the camber angle may be greatly varied from its standard state. In this case, for example, the running stability in cornering which is other than the running stability in straight, is conversely greatly lowered. It is important to judge the running stability performance of the tire even for carrying out the method described in Patent Document 1. Nevertheless, Patent Document 1 does not suggest a technique of judging the running stability performance of the tire.
Moreover, Patent Document 1 takes notice of the deformation of the tire, which is generated when the tire passes over the convex portion of the road surface while the vehicle is running straight, to provide the method and the device described therein under the idea that the variation of lateral force generated in the tire, which is caused by the deformation, and the running stability of the vehicle when the vehicle is running straight are associated with each other. In vehicle cornering, the load applied to the tire greatly varies due not only to the deformation generated by the tire passing over the convex portion of the road surface but also to an extremely small rolling behavior of the vehicle, which is caused by the relation between a centrifugal force and a suspension, stick-slip caused on a ground-contact surface of the tire, a resonance phenomenon between the suspension of the vehicle and the tire, and the like. The method and the device described in Patent Document 1 do not suggest a method of obtaining the running stability of the vehicle in cornering.
Further, according to the method described in Patent Document 1, the vehicle is placed on the cornering test device, and the tire is evaluated based on the combination of the single tire and the suspension. Therefore, the result of evaluation of the tire adversely varies depending on the suspension of the vehicle. Further, a laborious task for placing the vehicle on the cornering test device is required. Thus, the labor and the time required for the test are enormous, resulting in extremely high test cost.